1. Field of the Invention
This invention relates in general to electrical submersible subsea booster pumping systems, and in particular to reducing the temperature of a subsea submersible electric pump motor through heat exchange with a frame structure having a non-heat exchange related primary function.
2. Description of the Related Art
Electrical submersible pumps (“ESP”) are used for pumping high volumes of well fluid, particularly in wells requiring artificial lift. The ESP typically has at least one electrical motor that normally is a three-phase, AC motor. The motor drives a centrifugal pump that may contain a plurality of stages, with each stage comprising an impeller and a diffuser that increases the pressure of the well fluid. The motor has a housing that is filled with a dielectric lubricant or oil that both provides lubrication and aids in the removal of heat from the motor during operation of the ESP. A seal section is typically located between the pump and the motor for equalizing the pressure of the lubricant contained within the motor with the hydrostatic pressure of the well fluid on the exterior.
The ESP is typically run within the well with a workover rig. The ESP is run on the lower end of a string of production tubing. Once in place, the ESP may be energized to begin producing well fluid that is discharged into the production string for pumping to the surface.
During operation, the temperature of the oil in the motor of the ESP increases due to mechanical friction and electrical inefficiencies. According to most conventional designs, internal motor heat is dissipated by passing the produced (pumped) fluid over an outer surface of the motor housing which is in heat conductive contact with the stator of the motor. As such, a higher fluid velocity of the produced fluid around the motor, or a lower fluid temperature, can lead to increased heat removal from the motor.
One of the most important properties of the motor oil is to lubricate the bearings and thrust bearing of the motor. The oil is also generally vital in dissipating heat from the bearings and thrust bearings in order to help maintain the motor within its rated temperature, and thus maintain motor reliability. Rejection of heat from the oil to the surrounding well fluid, however, is usually limited due to the well fluid's high temperature, and also its poor heat transfer characteristics due to its high viscosity.
An increased temperature of the motor oil due to failure to adequately dissipate heat may lead to low performance or premature failure of the motor. U.S. Patent Publication No. 2010/0329908 by Martinez et al., titled “Heat Exchanger for ESP Motor,” commonly assigned to the same assignee, describes an advancement in motor cooling technology which describes an externally mounted heat exchanger to serve ESP equipment installed on the seabed. A hot oil line connects the base of the ESP motor with the externally located heat exchanger, allowing hot motor oil to be circulated through coils in a heat exchanger which are externally exposed to seawater. The heat from the oil is rejected to the seawater and the cooled oil is reintroduced to the motor via a cooled oil line that communicates with the seal section. The heat exchanger arrangement reduces the temperature of an ESP motor, thus allowing the motor to operate longer and more reliably.
Recognized by the inventors, however, is that it would be beneficial as to both capital cost and heat exchange efficiency to utilize existing structures adjacent ESP motor exposed to relatively cold seawater to perform the function of an external heat exchanger—i.e., to provide improved motor cooling by circulating oil or lubricant out of the motor to cool down the motor temperature, to thereby allow the motor to operate at a lower temperature that may translate to an extended life and increased reliability of the motor, without the need for a separate dedicated heat exchanger or the associated space/real estate taken by such separate dedicated heat exchanger.